The present invention relates generally to aircraft gas turbine engines, and, more specifically, to vectoring exhaust nozzles therefor.
A typical high performance, augmented gas turbine engine includes a varying area converging-diverging exhaust nozzle which is axisymmetric about a longitudinal or axial centerline axis. The nozzle includes a plurality of circumferentially adjoining primary exhaust flaps joined in turn to a plurality of circumferentially adjoining secondary exhaust flaps. The secondary flaps are joined by corresponding outer compression links to a common stationary casing also supporting the primary flaps.
This assembly is articulated in the manner of four-bar linkages to vary exhaust flow area, designated A8, at the nozzle throat between the primary and secondary flaps, and for varying the flow area of the nozzle outlet, designated A9, at the downstream end of the secondary flaps. Suitable linear actuators such as hydraulic actuators are circumferentially spaced apart around the casing and have respective output rods joined to the nozzle for pivoting the primary flaps to control the throat area and in turn control the outlet-to-throat area ratio.
In order to increase the maneuverability of aircraft powered by augmented gas turbine engines, vectoring exhaust nozzles are being developed. In U.S. Pat. No. 4,994,660, assigned to the present assignee, an Axisymmetric Vectoring Exhaust Nozzle (AVEN.RTM.) is disclosed. In this type of nozzle, a primary actuation ring surrounds corresponding cams on the outboard surfaces of the primary flaps and is operatively joined to a plurality of primary linear actuators which control its axial position perpendicular to the axial centerline axis of the nozzle. The outer links in this nozzle are joined to a secondary actuation ring which in turn is joined to a plurality of secondary linear actuators mounted to the casing.
During operation, axial translation or slide of the primary ring controls the pivoting of the primary flaps and in turn the nozzle throat area. The secondary ring may also slide axially to independently control pivoting of the secondary flaps, and in turn control both the outlet area and the area ratio. Furthermore, the secondary ring may be tilted in space to effect pitch or yaw, or both, in the secondary flaps to effect nozzle vectoring in which the engine exhaust is discharged at a slight angle from the engine centerline axis as opposed to coaxially therewith as in conventional non-vectoring exhaust nozzles.
Since the secondary flaps are vectorable they substantially increase the complexity of the nozzle design and its implementation. For this reason, many additional patents have been granted on various features of the AVEN.RTM. exhaust nozzle in behalf of the present assignee. These patents relate to both the mechanical details of the nozzle and the control systems therefor.
Since a plurality of circumferentially adjoining secondary flaps are utilized in the nozzle, suitable inter-flap seals must also be provided for preventing flow leakage between the flaps as the flaps are positioned through a suitable range of vectoring. This range, however, is limited to avoid inter-flap flow leakage or undesirable distortion of the various components.
Furthermore, the control system for the vectorable nozzle is being developed for a digitally programmable controller to control the actuators in feedback closed loops. The nozzle controller typically includes limiting values to prevent excess vectoring of the nozzle within the mechanical capabilities of the nozzle components. And, the nozzle controller must be sufficiently fast to process the required data in real time for the extremely fast maneuvering of the nozzle and the aircraft being powered therewith.
The complexity of the nozzle is further increased by using more than three actuators for the secondary actuation ring to provide redundancy. Redundant hardware requires precise control of the fourth or more actuators to prevent opposition with the initial three actuators which define the plane of the secondary ring. In some designs, it may be desirable to employ two redundant secondary actuator systems, with each system having three actuators. The six actuators must therefore be controlled in unison to prevent opposition load therebetween and to ensure that all the actuators operate synchronously.
Synchronous operation of the many actuators is yet further complicated in view of the redundant position sensors or detectors incorporated therein. Typical feedback control requires the measurement of output stroke of the individual actuators which is compared in the controller with the corresponding demand signal therefor, with the difference between the demand and measured strokes being driven to a minimum or zero value in a closed loop. Accurate feedback measurement is therefore important to the coordinated operation of the several actuators.
More specifically, considerable expense and initial calibration is required in the manufacture of the individual actuators to ensure that the redundant position detectors therein not only provide equal output with each other, but equal output with the cooperating actuators controlling thrust vectoring. A typical position detector is in the form of a Linear Variable Differential Transformer (LVDT) which typically includes an elongate coil having an output voltage which varies linearly in response to the axial stroke of the output rod of the actuator. During initial manufacture, therefore, the LVDTs must be carefully aligned in the individual actuators for obtaining equal linear response therefrom between minimum and maximum values during operation.
Furthermore, significant time and expense is also required to assemble the individual actuators to the corresponding vectoring ring for initially positioning the ring square or perpendicular to the centerline axis of the nozzle so that pure axial slide of the ring, and tilting thereof for effecting both pitch yaw of the ring may be accurately obtained by the coordinated stroke of the individual actuators joined thereto.
The precision manufacture and assembly of the several secondary actuators attached to the secondary actuation ring is required in view of the predetermined and fixed control algorithms stored in the nozzle controller which controls operation of the nozzle. Variations in initial calibration of the LVDTs in the several actuators, and variations in assembly of the actuators with the secondary actuation ring will therefore effect variations in performance of the vectoring nozzle which is difficult, if not impossible, to correct in the controller itself.
Accordingly, it is desired to provide an improved control system for the vectoring actuators in an exhaust nozzle which automatically calibrates for uncalibrated or miscalibrated actuators in the assembled nozzle actuation system.